Fin roll stabilizers are commonly mounted to the bottom of a vessel below the waterline, usually close to the turn of the bilge. These fins rotate about an axis that is perpendicular to the lengthwise axis of the vessel. The stabilizers are generally aligned parallel to the lengthwise axis of the vessel and rotation of these fins reduces roll of the vessel. The fin roll stabilizers act in some ways that are similar to ailerons on an airplane.
Traditional fin roll stabilizers for marine vessels are powered hydraulically. In order to create a functioning stabilizing system, a complicated setup of hydraulic plumbing, valves, cylinders and pumps are needed to operate and control the stabilizer. In addition, a reservoir must be provided to supply the pumps with hydraulic fluid and a cooling system and filter is needed to prevent overheating and to keep the fluid clean. The hydraulic system components all need connection with hose or pipe which can run long distances within a vessel and be difficult to install and require maintenance.
Electrical sensors and controls are then needed to operate various valves within the hydraulic system to allow the stabilizers to move through varying degrees of rotation that depend on vessel speed and wave conditions that are causing the vessel to roll.
The hydraulic system also comes with added environmental concerns associated with hydraulic oil because a leak in the system can bleed in to the bilge of the vessel and be pumped out with bilge water into the ocean or other body of water where the marine vessel is operating.
The hydraulic fluid is often put under high pressure such that the associated parts within the system must be designed to withstand that substantial pressure. This adds cost to the system. In addition, air in the hydraulic system can cause control problems and when the air bubbles collapse they generate intense localized heat in the hydraulic oil leading to system overheating and early hydraulic component failure. Air in the system can also cause objectionable noise and vibration transmitted throughput the vessel via the interconnecting hydraulic piping.
Another concern of a shipboard hydraulic system is fire. At higher pressures the hydraulic system is more prone to leaks and when a leak occurs the oil can mist or spray onto hot surfaces in the vessel's machinery spaces or be vaporized, and when exposed to a source of ignition vaporized oil causes fire.
One alternative to hydraulics has been direct drive electric motors that rotate the fin shaft. Stabilizer fins often require high torque to operate such that a direct drive motor needs to be relatively large and heavy. A reduction gear between the motor and fin would reduce the motor size, but the gear arrangement and motor together adds a significant height to the stabilizer and may not fit in the confined space within the hull.
In addition, failure of part of the stabilizer system can result in a crippled vessel because the fins may be in fixed positions that make it difficult to move the vessel under power. For example, if the fin is at 20 degrees to the lengthwise axis of the vessel, moving the vessel through the water will cause an un-desired list and fin drag when under power.